The present invention relates to the arts of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with gamma cameras and will be described with particular reference thereto. It is to be appreciated that the present invention is applicable to positron emission tomography (PET), whole body nuclear scans, and/or other like applications.
Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads, typically including collimators, are placed adjacent to a surface of the subject to monitor and record emitted radiation. Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. In single photon emission computed tomography (SPECT), emission radiation is detected by a single collimated detector. In positron emission tomography (PET), data collection is limited to emission radiation that is detected concurrently by a pair of oppositely disposed heads. The monitored radiation data from the multiplicity of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems with these imaging techniques is that photon absorption and scatter by portions of the subject or subject support between the emitting radionuclide and the detector heads distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogeneous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate SPECT and PET radiation attenuation measurements, a direct, transmission radiation emission measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Attenuated radiation rays are received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into an image representation using conventional tomography algorithms. Regional radiation attenuation properties of the subject and the support, which are derived from the transmission computed tomography image, are used to correct or compensate for radiation attenuation in the emission data.
PET measurements are typically made at incrementally stepped locations. One difficulty resides in optimizing the sampling of both the PET and SPECT emission data, and the transmission data so as to reduce overall scan time. Typically, PET emission data is acquired with greater resolution than transmission data (i.e., approximately 2-4xc2x0 per step for collecting emission data versus about 5-8xc2x0 per step for collecting transmission data). The total time to perform such a scan is composed of the time to actually acquire data at each angular orientation and the time to mechanically move the gantry from one angular orientation to another and stabilize it at the new orientation.
In a PET scan, the data is typically collected at angular increments of the desired resolution over the first 180xc2x0 of revolution. For 3xc2x0 of resolution, the data is collected at 60 steps each 3xc2x0 apart. When using transmission radiation in a fan beam pattern and a 360xc2x0 reconstruction algorithm, transmission data can be collected at 30 additional steps each 6xc2x0 apart (and the corresponding emission data discarded) or both can be collected at 60 additional steps each 3xc2x0 apart. After each indexing, there is a significant wait time while the physical position of the heads stabilizes. A wait time for stabilization of only 2-4 seconds per step adds 4-8 minutes to a 120 step scan.
The present invention contemplates a new and improved data sampling technique for gamma cameras which collect emission data and transmission data simultaneously which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of diagnostic imaging using a nuclear medicine gamma camera includes placing a subject in a subject receiving aperture and injecting the subject with a radiopharmaceutical. At least one radiation source and a plurality of radiation detectors are positioned about the subject receiving aperture such that the radiation source is across the subject receiving aperture from a corresponding radiation detector. Radiation from the radiation source is directed toward the corresponding radiation detector. The radiation source and radiation detectors are incrementally rotated about the subject receiving aperture by a predetermined step size throughout a first 180xc2x0 of rotation about the subject receiving aperture. The radiation source and radiation detectors are then offset by one-half of the predetermined step size and incrementally rotated about the subject receiving aperture by the predetermined step size throughout the remaining 180xc2x0 of one complete rotation about the subject receiving aperture. Radiation emitted by the injected radiopharmaceutical and transmitted by the radiation source is detected by the plurality of radiation detectors at each angular orientation corresponding to each step of incremental rotation about the subject receiving aperture. Emission projection data and transmission projection data are generated and reconstructed into a volumetric image representation.
In accordance with another aspect of the present invention, a positron emission tomographic camera includes a rotating gantry which defines a subject receiving aperture. A plurality of radiation detectors heads are movably attached to the rotating gantry such that the detector heads rotate about the subject receiving aperture with rotation of the rotating gantry. A rotational drive steps the plurality of detector heads around the subject receiving aperture in even steps where the steps in one half rotation are offset from the steps in the opposite half rotation by one half of the even step. At least one radiation source is mounted to at least one detector head such that transmission radiation from the radiation source is directed toward and received by a corresponding detector head positioned across the subject receiving aperture from the radiation source. An angular position sensor senses the angular position of the gantry and each detector head attached thereto as the gantry rotates about the subject receiving aperture. A reconstruction processor reconstructs the emission and transmission data into a volumetric emission image representation.
In accordance with another aspect of the present invention, a method of sampling transmission radiation data and emission radiation data for use in diagnostic imaging includes selecting a rotation increment for effective emission data imaging resolution. A radiation source and radiation detectors are incrementally rotated about a subject receiving aperture by a predetermined step size over a first 180xc2x0 of one complete rotation about the subject receiving aperture. Further, the radiation source and radiation detectors are incrementally rotated about the subject receiving aperture by the predetermined step size over a remaining second 180xc2x0 of one complete rotation offset by a half step relative to the first 180xc2x0 of rotation. Radiation emitted by the radiopharmaceutical and the radiation transmitted by the radiation source are detected at each stepped angular orientation corresponding to each incremental rotation about the subject receiving aperture.
In accordance with a more limited aspect of the present invention, the sampling method includes interleaving emission radiation data detected during the second 180xc2x0 incremental rotation with emission radiation data detected during the first 180xc2x0 incremental rotation.
One advantage of the present invention is that it reduces gantry dead time.
Another advantage of the present invention is that it reduces by approximately one-half the number of steps for emission and transmission data acquisition as conventional sampling methods.
Another advantage of the present invention is that it provides greater resolution for PET emission than is required for transmission data.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment.